The present invention relates to a method for fabricating an image sensor; and, more particularly, to a method for fabricating an image sensor that will effectively prevent contamination of the surface of a color filter array.
Conventional image sensor include both charge coupled device (CCD) image sensors and complementary metal oxide semiconductor (CMOS) image sensors. The basic technology used to form the CMOS image sensor is common to both types of sensors.
The CMOS image sensor comprises a photo detector for detecting light and a logic circuit for converting the detected light into an electric signal representing data regarding the detected light. Although efforts have been made to increase the fill factor of the image sensor and thereby increase the sensor sensitivity, further increases in the fill factor are limited because the associated logic circuitry cannot be completely removed. The fill factor, sometimes referred to as the aperture efficiency, is the ratio of the size of the light-sensitive area to the size of the total pixel size. Accordingly, in order to increase the sensitivity of the light, a micro lens formation technology has been used to converge and focus the incident light onto the photo detector by changing the path of the light that reaches the lens on the outside of the photo detector. In order for the image sensor to detect and provide a color image, it typically must include both a photo detector for receiving the light and generating and accumulating charge carriers and a color filter array (CFA), i.e., a plurality of color filters sequentially arranged above the photo detector. The CFA typically uses one of two alternative three-color primary configurations, either red R, green G and blue B (RGB) or yellow Y, magenta M and cyan C (CMY). A plurality of micro-lenses may be positioned above the color filter array to increase the photo-sensitivity of the image sensor.
Although one inner polysilicon and three outer metal layers have been used to form the interconnector structures in the conventional image sensor, the third metal layer is typically completely removed from above each detector pixel, a passivation layer is deposited on each pixel and then the color filter array is fabricated.
Accordingly, in a conventional image sensor, increased thickness between a photodiode of each pixel and the corresponding color filter array results in a larger loss of the available light signal. Further, scum resulting from the under-development of the organic films used in forming the color filter array will tend to degrade the features of the resulting pixels
Referring to FIG. 1, there is illustrated a conventional method for fabricating an image sensor. A field insulating layer 12 is formed on a silicon substrate 11 in order to isolate two neighboring pixels; a photodiode 13 is formed by implanting and/or diffusing impurity; and an inner connection polysilicon 14 is then formed on the field insulating layer 12. After a pre-metal dielectric (PMD) layer 15 for transmitting the light is deposited on the inner connection polysilicon 14, a first interlayer insulating layer 16 for insulating two neighboring metal lines is deposited. After a first metal line 17 and a second interlayer insulating layer 18 are sequentially deposited on the first interlayer insulating layer 16, a second metal line 19 is constructed on the second interlayer insulating layer 18 so as to be opposite to the first metal line 17.
After a third interlayer insulating layer 20 is formed to insulate neighboring metal lines is formed on the second metal line 19, a third metal layer is deposited. This third metal layer will be only used to formulate metal lines for the peripheral circuits and will typically be removed completely from the light sensing areas of the unit pixel. After the third metal layer is removed, a high temperature process is used to form an oxide layer, a nitride layer or a multi-layer film of oxide and nitride layers, on the third interlayer insulating layer 20 to provide a device protection layer 21 that will protect the device from moisture and mechanical damage.
After a color photoresist layer for providing color sensitivity is deposited on the device protection layer 21, developing processes are performed to generate the color filter array, i.e., an array of a blue color filter 22, a red color filter 23 and a green color filter 24. The red color filter 23 is thicker than the blue color filter 22 by a predetermined thickness and the green color filter 24 is, in turn, thicker than the red color filter 23 by another predetermined thickness. The differences in thickness between the various filters produces a color filter array having a stepped structure that tends to be contaminated by scum deposits (A, B, C). The scum is not primarily deposited on the oxide device protection layer 21, but is more typically found on photoresist layers that form the color filter array. For example, in the process of sequentially forming the color filter array of the blue color filter 22, the red color filter 23 and the green color filter 24, the scum deposits A and B produced during formation of the red color filter 23 or the green color filter 24 may remain on the blue color filter 22. Similarly, the scum deposits produced during the formation of the blue color filter 22 may remain under the red color filter 23, while scum originating from the green color filter 24 may remain on the red color filter 23. However, although the scum originating from the blue color filter 22 may remain on the device protection layer 21, because the device protection film 21 and the blue color filter 22 are formed from dissimilar materials, scum is not typically found on the oxide layer. Further, because the scum remaining on the red color filter 23 originates primarily from the green color filter 24, the upper regions of the color filter array may frequently suffer from scum deposits.
Three color filters in the color filter array are necessarily stepped as a result of the three fabricating processes of the color filter array.
If the stepped color filter array is planarized, the last formed color filter in the CFA is thicker than the previously formed color filters. As a result, the final thicknesses typically increase in sequence from the blue color filter 22, to the red color filter 23, and finally to the green color filter 24.
An over coating material (OCM) 25 is formed on the color filter array, wherein the over coating material 25 is used to perform a flattening or planarization process with respect to the stepped color filter array. A plurality of micro-lenses 26 is then formed on the over coating material 25 above the color filter array.
As a result of this process, the conventional method for fabricating a color filter array typically results in scum from the photoresist material being found on the color filter array.
It is, therefore, an object of the present invention to provide a method capable of enhancing the uniformity of the color filter array and removing the scum on the surface of the color filter array to improve the yield ratio of the device.
In accordance with a preferred embodiment of the present invention, there is provided a method for fabricating an image sensor, comprising the steps of: (a) formulating devices on a substrate and forming a first and a second interlayer insulating layer and a first and a second metal pattern, wherein the devices include a photodiode; (b) forming an optical shielding layer on the second interlayer insulating layer; (c) using the optical shielding layer as a mask to perform a dry etch of the second inter layer insulating layer to remove a predetermined thickness, thereby producing a plurality of grooves; (d) filling the plurality of grooves to form a color filter array on the second inter layer insulating layer; (e) utilizing the optical shielding layer as a polishing stop layer to perform a chemical mechanical polishing (CMP), thereby planarizing the color filter array; and (f) sequentially depositing both a low temperature layer and a device protection layer.